In 1952, he joined the Hebrew University faculty, lecturing in botany. From 1952 to 1953, he was a research fellow at the
University of Cambridge and in 1956 at
Harvard University. He became an associate professor at the Hebrew University in 1960 and a full professor in 1965, serving as
Dean of the faculty from 1964 to 1966 and pro-rector from 1969 to 1970. Following the
Six Days War in 1967, Fahn assisted in the restoration of the botanical gardens at the University's
Mount Scopus campus. He also served for many years as head of the Forestry Department of the
Volcani Institute of Agricultural Research. Fahn retired in 1985. His books on the anatomy of plants have become basic texts in the field of botany. ==Awards and recognition==